From the viewpoint of global environmental protection, it is always an important issue for the automotive industry that in order to reduce CO2 emissions, automotive fuel efficiency is improved by reducing the weight of automobile bodies with the strength thereof maintained. The reduction in thickness of steel sheets, which are materials for automotive parts, by strengthening the steel sheets is effective in reducing the weight of automobile bodies with the strength thereof maintained. Most of automotive parts made from steel sheets are formed by pressing, burring, or the like. Therefore, high-strength steel sheets used as materials for automotive parts are required to have desired strength and, furthermore excellent formability. In recent years, ultra-high-strength steel sheets with a tensile strength of 1,300 MPa or more have been increasingly used as framework materials for automobile bodies. Upon forming the ultra-high-strength steel sheets, excellent stretch-flange formability and bendability are necessary. Against such a background, various ultra-high-strength steel sheets excellent in formability have been developed. However, increasing the content of an alloying element in steel for the purpose of achieving high strength causes the in-plane variation of formability, particularly stretch-flange formability. As a result, there is a problem in that materials having sufficient properties cannot be provided. Thus, it is very important to solve the problem.
Patent Literature 1 discloses a technique relating to a high-strength cold-rolled steel sheet excellent in elongation and stretch-flange formability. However, the in-plane variation of stretch-flange formability is not evaluated. Therefore, it is uncertain that the high-strength cold-rolled steel sheet has sufficient in-plane stability of stretch-flange formability.
Patent Literature 2 discloses a technique relating to a high-strength galvanized steel sheet excellent in bendability. However, the strength thereof is low, about 1,000 MPa in terms of tensile strength (TS), and stretch-flange formability is not taken into account. There is room for improvement.
Patent Literature 3 discloses a technique relating to a high-strength galvanized steel strip with few variations in strength in the steel strip. However, variations in stretch-flange formability and bendability are not taken into account. There is room for improvement.